The invention is in the field of photoflash lamps of the miniature type in which a pair of inleads carry a filament or other electrical ignition means inside a bulb having a volume less than two cubic centimeters and containing combustion material such as shredded metal foil and a combustion-supporting gas such as oxygen. Miniature flash lamps of this type are disclosed in U.S. Pat. Nos. 2,982,119 to Robert Anderson and 3,506,385 to Kurt Weber and George Cressman. The inleads extend into the bulb through a seal at the base of the bulb, and the lamps are conventionally positioned base-down when flashed in flashcubes and multiple flash arrays.
When such a photoflash lamp is flashed, the shredded metal foil burns in an oxygen atmosphere and produces an intense flash of light. Occasionally, some remnants of metal will remain in a flashed lamp and tend to bridge across the inleads, causing an electrically shorted flashed lamp. Due to the small size of miniature flash lamps, it is not feasible to provide an internal glass stem to seal and support the inleads within the bulb as had been the case with previous large-bulb flash lamps because such an internal stem would be difficult to construct in such a small bulb and would considerably reduce the amount of space available for the oxygen.
In miniature flash lamps of the type containing a glass support bead through which the inleads are sealed, unburned foil remnants can fall on top of the bead and cause a short between the inleads. In miniature flash lamps of the type not having a support bead, unburned foil remnants can fall between the inleads to the bottom of the lamp and cause a short.
Such shorting of a flashed lamp is undesirable because it will cause a short circuit on the battery or other power source until the lamp is removed from the circuit. Many lamp flashing circuits alleviate the problem by providing a quick make-and-break synchronizing switch which, after closing to apply current to the filament to cause a lamp to flash, immediately opens to provide an afterflash open circuit even though the flashed lamp may be shorted. Such switch circuits are customarily employed for flashing individual flash lamps, and the four flash lamps of a flashcube.
Firing circuits, employing transistor devices or other types of switches, have been developed for sequentially flashing, one at a time, a plurality of flash lamps in the form of a multiple-flash array. All of the lamps at the front side of the array are flashed in turn without moving or turning the array. An example of such a multiple-flash array, having five flash lamps on each side, is described in U.S. Pat. Nos. 3,598,984 to Stanley Slomski and 3,598,985 to John Harnden and William Kornrumpf. An example of an electronic firing circuit for a multiple-flash lamp array is described in U.S. Pat. No. 3,676,045 to Donald Watrous and Paul Cote. The circuit functions, basically, by sequencing past open-circuit flashed lamps, and applies a firing pulse to the first lamp in the array having a proper filament resistance (of low value such as 0.6 ohms). An "open-circuit" flashed lamp, by comparison, normally has a resistance, between its lead-in wires, of a few hundred ohms or greater. If a lamp, upon being flashed, becomes a "shorted" lamp (as described above and having a low resistance of, for example, several ohms or less), the circuit will thereafter apply each succeeding firing pulse to the shorted flashed lamp, and no further good lamps connected in the circuit can be flashed. Similar difficulty can occur with other types of sequencing circuits such as those using heat-sensitive switches. Thus, the need is evident for a flash lamp design that will almost invariably provide an open circuit upon flashing.
The above-referenced patent application is directed to a solution of the lamp-shorting problem for the type of flash lamp in which the inleads burn or melt back from the filament due to heat of combustion when the lamp is flashed. A glass sleeve is positioned around an inlead, or an electrically insulative member is positioned between the inleads, in a manner to increase the electrical path distance between the burned-back inleads when the lamp is flashed.